1. Field of the Invention
The present invention relates to a radar device that calculates target information on the basis of a reflected signal when a transmitting signal (electric wave) is emitted to a target object (hereinafter, referred to as “target”), and the reflected signal based on the transmitting signal from the target is received by a plurality of receiving antennas.
2. Description of the Related Art
Up to now, as a radar device for calculating a distance to a target or a relative velocity thereto, there has been known a frequency modulated radar device that transmits a transmitting signal having a successively modulated frequency to the target, and receives a reflected signal from the target to calculate a distance to the target and a relative velocity thereto.
As a method of calculating a target direction in the radar device of this type, there has been known a method of calculating a direction of the target by mechanically rotating transmitting means to scan the transmitting signal. Further, there has been known a super-resolution arrival direction estimating process such as a multiple signal classification (MUSIC) method that calculates a direction of the target by outputting a transmitting signal without mechanically rotating the transmitting means and subjecting a received signal that has been received by an array antenna having a plurality of channels to digital signal processing (for example, refer to R. O. Schmidt, “Multiple Emitter Location and Signal Parameter Estimation,” IEEE Trans. Vol. AP-34, No. 3, March 1986, pp. 276 to 280).
In the MUSIC method, a correlation matrix of a peak frequency spectrum is computed, the correlation matrix is subjected to eigendecomposition, an angle spectrum is computed from an eigenvector, and a direction of the target is calculated according to the angle spectrum.
The super-resolution arrival direction estimating process represented by the above-mentioned MUSIC method cannot be applied as it is to a land mobile communication or the like which is very high in correlations among incoming waves because the process assumes that the respective incoming waves is uncorrelated with each other. In general, in order to suppress the correlations among the incoming waves, it is desirable to increase the number of received signals (number of snapshots) used for generation of the correlation matrix.
Under the above-mentioned circumstances, in the general radar device, the correlation matrix is obtained for each measurement, and hence there is used a temporal averaging method in which a plurality of the correlation matrixes are added together to ensure the number of snap shots. More specifically, the direction of the target is calculated on the basis of a summed correlation matrix in which a correlation matrix generated from an up-period peak frequency spectrum and a correlation matrix generated from a down-period peak frequency spectrum are added together.
However, in the case where at least one of the up-period peak frequency and the down-period peak frequency falls within a given frequency range, when the correlation matrix generated from the peak frequency spectrum having a peak frequency within the given frequency range is added to the correlation matrix generated from the peak frequency spectrum having a peak frequency outside of the given frequency range, there arises such a problem that the direction of the target cannot be accurately calculated, or the number of targets is miscalculated.
This is because, when noises generated in a transceiver or an A/D converter, an offset voltage caused by the variation or the temperature characteristics, a reflected signal from a stopping target, or the like is superimposed on the received signal, and the superimposed signal is analyzed in frequency, the frequency is reflected on a portion within the given frequency range. Accordingly, the addition of the correlation matrix generated from the frequency spectrum having a frequency within the given frequency range disenables the direction of the target to be accurately calculated, or causes the number of targets to be miscalculated.
Further, when a plurality of targets exist, a number of up-period peak frequencies and down-period peak frequencies corresponding to the number of targets appear. In order to calculate distances R among the respective targets and relative velocities V, it is necessary to combine the up-period peak frequency fbu and the down-period peak frequency fbd together for each target.
However, after the combination has been completed, in the case where any one of an up-period peak frequency fbu_a and a down-period peak frequency fbd_a of a target a overlaps an up-period peak frequency fbu_b and a down-period peak frequency fbd_b of a target b, when a correlation matrix generated from the peak frequency spectrum of the overlapping peak frequency is added to a correlation matrix generated from the peak frequency spectrum of the nonoverlapping peak frequency, there arises such a problem that directions of the targets a and b cannot be accurately calculated, or the numbers of targets a and b are miscalculated.
This is because, in the case where the peak frequencies caused by the plurality of targets having different distances and different relative velocities overlap each other, when phases of the peak frequencies are calculated, phases of the reflection waves due to the plurality of targets are combined together, respectively, whereby the precise correlation matrix cannot be calculated. Accordingly, when the correlation matrix generated from the frequency spectrum of the peak frequencies is added, the direction of the target cannot be accurately calculated, or the number of targets is miscalculated.